Recently, wavelength division multiplexing networks using an OTN (Optical Transport Network, ITU-T.G709) technology have been put to practical use in order to increase the capacity of networks. In such networks, a protection scheme for fast fault recovery is needed for occurrence of a fault.
For ring networks in which wavelengths are not multiplexed as in a conventional Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH), protection techniques such as Bidirectional Line Switched Ring, Telcordia GR-1230-CORE (BLSR), Multiplex Section Shared Protection Ring, ITU-T G.841 (MSSPRING), and others are used.
These protection technique performs fault recovery by turning a path back at both nodes of a fault link. When this technique is adapted to a ring network in which wavelengths are multiplexed, a protection ring which is configured in each wavelength channel independently operates.
However, a process of turning a path back in each wavelength channel increases a processing load on an optical transmission apparatus, and therefore fast recovery is difficult. To deal with this problem, think of combining the ITU-TG. 808.1 group protection scheme. This approach sets a plurality of wavelength channels to one group, and when faults occur in one or more wavelength channels in a group, paths using all wavelength channels of the group are collectively switched, and therefore fast recovery is realized.
There are proposed a method for detecting a fault occurring in each optical wave path at its path end and performing fault recovery on an optical wave path basis in a wavelength division multiplex (WDM) transmission network, and a transmission apparatus for implementing the method (see, for example, Japanese Laid-open Patent publication No. 2000-209152).
However, a method for switching paths in a group of a plurality of wavelength channels has a drawback in that, when there is a plurality of portions in which a fault has occurred in a part of wavelength channel in the ring, path switching is performed in each fault portion, and therefore all of the paths including normal paths may be cut off.